ADS7142: Autonomous Mode with Threshold Monitoring

Qmshi,

Sorry for the long delay on this. I hope the explanation below helps.

1. The Alert pin uses an open drain configuration to drive the level. When Alert transitions from high to low, the transistor turns on and the resistance to ground will be very low. This will pull the Alert pin from high to low very quickly.

2. When the Alert pin is transitions from low to high, the transistor is turned off. To transition from low to high will be very slow as the bus capacitance needs to charge through the very large 2.2Mohm resistor. The actual bus capacitance depends on the output capacitance of the Alert pin, the input capacitance of the microcontroller, and the parasitic capacitance of the Alert PCB trace. A rough estimate is about 10pF.  Thus, the time constant is RxC = 2.2Mohm x 10pF = 22us. In five time constants the bus capacitance is 99% charged, so the low to high transition is about 110us. You could measure this.

3. Alert is active low. So when an event that sets the Alert in an active state will be detected quickly because the high to low transition is very quick.

4. I assume that you are using the large pull-up to minimize current consumption. Keep in mind that the device will not draw any current on the Alert pin when the output is in a high state. The only time that power is dissipated from the Alert pin is when it is active low. Depending on how often this happens you may not see much power dissipation from this pin.

5. Normally, I2C pull-ups (SCL and SDA) are set to 4.7k. On the high end they can range to 49k, but this will limit communications speeds. For the Alert you just need to make sure that you can accept the long rise time for the low to high transition. I’m not sure why you want such a large Alert Pull-up resistance as you are likely dissipating much more power in the SCL and SDA line. I recommend you just keep the alert pull up consistent with SCL and SDA.